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Avalanche Forecast

Archived

Dec 12th, 2020–Dec 14th, 2020

Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.

Regions

North Rockies.

Sunny skies may be enticing, but there remains uncertainty as to whether a deeply buried weak layer is a problem in the region; use caution in steep terrain at higher elevations. New wind slabs are expected to form on Monday.

Confidence

Moderate - Uncertainty is due to the limited number of field observations.

Weather Forecast

SATURDAY NIGHT: Mostly clear skies, 20 km/h northwest wind, alpine temperature -14 C.

SUNDAY: Mostly clear skies, 20 km/h southwest wind, alpine temperature -15 C.

MONDAY: Mostly cloudy with light snowfall, accumulation 5 cm, 40 to 60 km/h west wind, alpine temperature -8 C.

TUESDAY: Cloudy with snowfall, accumulation 5 to 10 cm, 40 to 60 km/h southwest wind, alpine temperature -7 C.

Avalanche Summary

A few old slabs were reported around Torpy on Friday in this MIN. Given the looks of the slabs, they likely released quite a few days ago. Otherwise, no other avalanches have been reported recently.

While there have not been any recent reports of deep persistent slab avalanches, we can not rule out the possibility that this problem still lurks in parts of the region. Faceted grains above a crust near the bottom of the snowpack was the likely failure layer in a fatal avalanche near Pine Pass on November 28. With little recent information about this layer it is best to remain cautious around large, high consequence slopes, especially if they are thin and rocky.

Snowpack Summary

Snow depths are approximately 150 cm around Pine Pass, Torpy and McBride and 50-100 cm in the northeast around Tumbler Ridge. 

The alpine is heavily wind-affected and lower elevations are capped by a hard crust up to around 1300 to 1600 m. Sheltered areas at treeline may still have 5 to 20 cm of soft snow on the surface. Monday's strong wind will likely form new wind slabs in exposed terrain.

A widespread crust layer that may have sugary faceted grains above it can be found at the bottom of the snowpack. Uncertainty remains about if and where this layer is a problem. Recent observations from the McBride area suggest the snow is well-bonded to this crust. There is no recent information from northern parts of the region, such as Pine Pass and Tumbler Ridge, where this layer was a problem earlier this season.

Terrain and Travel

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.
  • Avoid thin areas like rock outcroppings where you're most likely to trigger avalanches failing on deep weak layers.

Problems

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.